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You mean how long the attacker wants to hold the secondary chain hidden? Nobody knows. The actual takeover in terms of hashing power can happen instantly.

That's what I wanted to know. Instant take over rings some alarm bells for me. Next question though. Is it technologically possible to prevent it from happening?

From a technical point of view, no. Technically speaking, this is a Bitcoin fork, which uses Bitcoin hashing power. So, technically speaking, it can happen.